In blood vessel-walls there is a monomolecualr negativelycharged protein layer that prevents proteins and uric acid[which is also N-containing ]sticking to it. MOreover there is a cotinuous pressure on each 'unit volume'.
But In joint-cavities as in some parts this pressure is less. So molecules can remain there for more time and if their ammount exeeds the solubility product of uric acid, it will form crystals. As there is no repusive layer [may be I'm unaware ] , these crystals adhere or get unable to move uhead through cavities due to size.
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0000 l.....lesser...........l
00_ _l....preshr............l_ _ _ _ _ _ _